So here we have the mirror “d0” made up of devices “d10”
and “d11”. Each of these devices can be addressed in the
file system as /dev/md/rdsk/d0 /dev/md/rdsk/d10 and /dev/md/rdsk/d11
respectively. The block devices are also available if you so desire.
While being able to address the underlying disk devices that make up
a mirror is interesting and potentially useful it is only useful if
you really know what you are doing.

Reading
from the mirrors is o.k. Writing and that includes just mounting the
file system is not. So if the device is idle you can do:

# cmp /dev/md/rdsk/d10 /dev/md/rdsk/d11
#

Which if it returns 01
gives you a feeling of confidence, although if you are this paranoid,
and I am, then ZFS is a much better bet.

For
example if the mirror contains a file system then mounting one side
of the mirror and making modifications is a really really
bad idea, even if the mirror is unmounted.
Once you have made such a modification you would have to make sure
the other side of the mirror had exactly the same change at the block
level propagated to it. Realistically the only way to achieve that
is for you to detach the other mirror and then reattach it so allow
it to resync. If you really know what you are doing
there are tricks you could do but I suspect those that really
know what they are doing would
not get into this mess in the first place.

1
If it does not then you have to look at how the mirror was
constructed before you start to worry. If you did “metainit d0
–m d10 d11” or have grown the metadevice then the
mirrors will never have been brought into sync. So only the blocks
that have been written to since the operation will correctly
comapare. Hence this is nothing to worry about. See I told you you
do really have to know what you are doing.